EP3179276B1 - Methods and devices for validating the synchnonization between a geolocalizing receptor and an emitting satellite - Google Patents

Description

The present invention relates to a method for validating synchronization between a geolocation receiver and a transmitting satellite during an acquisition phase of a navigation signal originating from this satellite, and an associated geolocation receiver.

The invention also relates to a method for validating synchronization between a geolocation receiver and a transmitting satellite during an acquisition phase of an augmentation signal comprising geolocation correction and integrity data for a predetermined satellite geolocation, and an associated geolocation receiver.

The invention lies in the field of satellite geolocation systems, known by the acronym GNSS (for “ Global Navigation Satellite System”).

In general, a GNSS system is composed of a plurality of satellites, or constellation of satellites, allowing a portable geolocation receiver to determine positioning information, in a terrestrial frame, also called position, speed and time information ( TVP).

There are currently several GNSS systems among which mention may be made in particular of the GPS system, the GLONASS system or even the GALILEO system, the commissioning of which is planned shortly.

The satellites of such a GNSS system are capable of transmitting radio signals comprising in particular navigation information.

Each navigation item of information generally includes data relating to the time of transmission by the satellite of the corresponding signal and to the current position of the satellite. In particular, the data relating to the current position of the satellite generally contain the almanac giving a rough position of the satellite and the ephemeris giving the exact current position of the satellite.

The navigation information is carried by a carrier wave and modulated by a spreading code specific to each satellite. Thus, the signals are transmitted by the satellites using a spread spectrum technique.

The geolocation receiver, also called a GNSS receiver, is able to receive the signals transmitted by the satellites and to extract the navigation information therefrom in order in particular to determine the distance to the transmitting satellite having transmitted the corresponding signal. This distance, also called pseudo-distance, is determined by analyzing the propagation time of the corresponding signal.

To determine the PVT positioning information, the receiver implements digital processing of the navigation information coming from at least three different satellites.

In practice, to have a more precise position, the receiver needs navigation information from at least four different satellites.

More precisely, to acquire the navigation information of a given satellite, the receiver implements two phases processing the signals originating from this satellite.

During an initial phase, called in the state of the art, acquisition phase, the receiver generates a local signal containing in particular a local spreading code presenting the image of the spreading code of the satellite.

As initially the receiver does not know its position, the local signal is not synchronized with the received signal. This means in particular that the local signal is offset in carrier frequency from the received signal by a value called the Doppler value, and that the spreading code of the received signal is delayed from the local spreading code by a value called the value delay.

Then, the receiver performs a search for a peak of the correlations between the local signal and the received signal by trying different Doppler and delay values.

When a peak is detected, the receiver determines the Doppler and delay values corresponding to this peak and from these values, launches a following phase, called in the state of the art, tracking phase.

During the tracking phase, the receiver regularly updates the Doppler and delay values, and extracts the navigation information from the signal transmitted by the satellite using in particular the local spreading code and the Doppler and determined delay.

At the end of the acquisition phase, it is considered that the receiver has synchronized with the transmitting satellite or even has "hooked" to this satellite, thanks to the detection of the correlation peak.

It sometimes happens that the receiver synchronizes its local signal corresponding to the satellite sought on the signal received from another satellite, which leads to an erroneous distance measurement, and therefore potentially to a false positioning.

In this case, it is a false synchronization or a false "hang-up", which is also called cross-correlation. In this case, the calculation of the positioning information of the receiver is distorted.

In particular, cross-correlation error arises when satellites transmit short periodic code GNSS signals, for example GPS L1 signals C/A (acronym for “coarse acquisition”), corresponding to a frequency of 1575.42 MHz, SBAS L1 C/A and GALILEO L1 BC.

A similar phenomenon also occurs when the signal received comes from a satellite transmitting position correction information, in a spatial augmentation system, called the SBAS system.

There exist in the state of the art various methods making it possible to avoid such false synchronization or cross-correlation.

The documents US2015/009065 A1 , US2012/134392 A1 And JP H11 118900 A describe cross-correlation detection methods.

One method, used conventionally, consists in checking the consistency between the position of the satellite calculated from the ephemeris contained in the navigation information and that calculated from the decoded almanacs. The almanacs contain the identifiers of all the transmitting satellites of the constellation, unlike the ephemeris. The inconsistency between these values therefore means a false synchronization.

Indeed, the ephemeris data of a satellite make it possible to estimate the position of this satellite with an accuracy of a few meters, but are transmitted only by the satellite itself and have a validity period limited to a few hours.

The almanac data for the entire constellation of satellites make it possible to estimate the position of each of the satellites roughly, to within a few hundred kilometers, but are transmitted by all the satellites of the constellation and have a duration of valid for several days.

Thus, if the difference between the position calculated from the ephemeris and the position calculated from the almanacs is greater than the average distance between transmitting satellites, it is estimated that there is an error, therefore a cross correlation.

In order to avoid false cross-correlation detection and to guarantee the integrity of the positioning information calculated by a GNSS receiver, the aeronautical standard RTCA (Radio Technical Commission for Aeronautics) DO-229 “Minimum Operational Performance Standards for Global Positioning System » imposes a complete decoding of the ephemeris data sets received twice and a comparison with the decoded almanac data for all the satellites.

For a given transmitter satellite, a complete set of ephemeris data is formed of a given number of ephemeris words, each having an associated rank and encoding information relating to the transmitter satellite, this information making it possible to calculate the position of the satellite transmitter. Ephemeris data is retransmitted periodically, and renewed at a given frequency, for example every two hours for GPS satellites.

The ephemeris words are transmitted in subframes of the transmitted signal, the transmission of all the words of an ephemeris requiring a plurality of subframes.

Thus, the classic method of validating the absence of cross-correlation takes a relatively long time, which is 48 seconds to 60 seconds for the GPS system and several minutes for the SBAS system.

The object of the present invention is to remedy this drawback.

To this end, according to a first aspect, the invention proposes a method for validating synchronization between a geolocation receiver with a transmitting satellite during a phase of acquisition of a navigation signal from this satellite according to claim 1 .

Advantageously, the invention makes it possible to validate the synchronization of the receiver with a transmitter satellite from only part of the ephemeris words received, and consequently to considerably reduce the time for confirming the absence or presence of false synchronization.

The synchronization validation method according to the invention may also have one or more of the characteristics of dependent claims 2 to 8, taken independently or in combination according to all the technically possible combinations.

According to another aspect, the invention relates to a geolocation receiver implementing synchronization with a transmitting satellite during a phase of acquisition of a navigation signal originating from this satellite according to claim 9.

• la figure 1 est une illustration schématique d'un système de géolocalisation ;
• la figure 2 est un synoptique des principales étapes d'un procédé de validation de synchronisation selon un premier mode réalisation de l'invention ;
• la figure 3 est un synoptique des principales étapes d'un procédé de validation de synchronisation selon un deuxième mode réalisation de l'invention ;
• la figure 4 est un synoptique des principales étapes d'un procédé de validation de synchronisation selon un troisième mode réalisation de l'invention.

Other characteristics and advantages of the invention will emerge from the description given below, by way of indication and in no way limiting, with reference to the appended figures, among which:

• there figure 1 is a schematic illustration of a geolocation system;
• there figure 2 is a block diagram of the main steps of a synchronization validation method according to a first embodiment of the invention;
• there picture 3 is a block diagram of the main steps of a synchronization validation method according to a second embodiment of the invention;
• there figure 4 is a block diagram of the main steps of a synchronization validation method according to a third embodiment of the invention.

There figure 1 illustrates a geolocation system 1 suitable for implementing the invention, in the context of the navigation aid for a mobile carrier 2, which is in the example of the figure 1 an aircraft.

Of course, the invention is not limited to this embodiment, and applies more generally to the geolocation of any mobile carrier.

The mobile carrier 2 is equipped with a geolocation receiver 12, or GNSS receiver, capable of receiving radio navigation signals from a plurality of transmitter satellites 4, 6, 8, forming part of a constellation of satellites of a GNSS geolocation system.

In a first embodiment, it is a constellation of GPS system satellites.

As a variant, it is a constellation of satellites of the GALILEO system, or of any other GNSS system.

In general, the mobile carrier 2 is capable of receiving radio signals from a GNSS geolocation system, capable of transmitting in predefined frequency bands.

Each of the satellites transmits radio navigation signals, also comprising ephemeris data consisting of a plurality of ephemeris words, making it possible to calculate a position of the transmitter satellite in a given terrestrial reference frame, with a given precision, for example a precision a few meters. Additionally, the ephemeris data contains information about the internal clock of the transmitting satellite.

In known manner, the satellites are capable of transmitting signals on several transmission frequencies, the transmitted signals having an associated periodic code.

For example, in the case of the GPS system, the GPS L1 C/A signals are transmitted at a frequency of 1575.42 MHz with a periodic spreading code of 1023 chips. C/A codes are accessible to everyone and widely used in radio navigation applications.

These satellites are also capable of transmitting on other frequencies, for example 1227.60 MHz (L2 signals) or 1176.45 MHz (L5 signals).

In particular, ephemeris data can be transmitted in L5 signals, with a periodic code of 10230 chips, allowing much greater immunity to cross-correlation than L1 signals.

In addition, as a variant, correction and integrity data of one or more satellites 10 of a constellation of geostationary satellites, according to the spatial precision augmentation system using geostationary satellites, called SBAS (for "satellite- based augmentation system”), which are also transmitted in these radio signals transmitted in the same predefined frequency bands.

Receiver 12 has multiple receive channels for receiving spatial precision augmentation signals from multiple satellites.

The GNSS receiver 12 comprises in particular a calculation device, comprising one or more calculation processors 14, capable of executing calculations and computer program code instructions when they are powered up. The calculation device also comprises one or more storage memories 16, capable of storing executable code instructions allowing the implementation of programs comprising code instructions capable of implementing the methods according to the invention.

In one embodiment, the computing device is a computer.

As a variant, the calculation device is an electronic device of the programmable logic circuit type, for example one or more electronic cards based on FPGA or ASIC.

In particular, almanac data and ephemeris data relating to each of the satellites 4, 6, 8, 10 considered are stored in memory 16.

• un module d'extraction de mots d'éphéméride du signal de navigation associé au satellite identifié dans la phase d'acquisition, au fur et à mesure de la réception dudit signal de navigation,
• un module de comparaison d'au moins une partie desdits mots d'éphéméride extraits à au moins un mot d'éphéméride de même rang mémorisé pour ledit satellite identifié et/ou pour au moins un autre des satellites émetteurs, et
• un module de validation ou non de la synchronisation avec ledit satellite émetteur identifié, en fonction du résultat de la comparaison, dès que les comparaisons effectuées permettent d'atteindre une probabilité de fausse alarme et/ou une probabilité de non-détection prédéterminée.

The calculation processor 14 implements modules for implementing a synchronization validation test for each identified transmitter satellite, not shown in the figure, comprising:

• a module for extracting ephemeris words from the navigation signal associated with the satellite identified in the acquisition phase, as said navigation signal is received,
• a module for comparing at least a part of said ephemeris words extracted with at least one ephemeris word of the same rank stored for said identified satellite and/or for at least one other of the transmitting satellites, and
• a module for validating or not the synchronization with said identified transmitter satellite, depending on the result of the comparison, as soon as the comparisons made make it possible to reach a probability of false alarm and/or a predetermined probability of non-detection.

• un module d'extraction de mots reçus du signal d'augmentation associé au satellite identifié dans la phase d'acquisition, au fur et à mesure de la réception dudit signal,
• un module de comparaison d'un mot reçu à au moins un mot reçu de même type reçu pour au moins un autre des satellites émetteurs,

et, un module de validation ou non de la synchronisation avec ledit satellite émetteur identifié, en fonction du résultat de la comparaison, dès que les comparaisons effectuées permettent d'atteindre une probabilité de fausse alarme et/ou une probabilité de non-détection prédéterminée.In addition, the calculation processor 14 implements modules for implementing a validation test for each satellite 10 transmitting a signal augmentation including geolocation correction and integrity data, including:

• a module for extracting words received from the augmentation signal associated with the satellite identified in the acquisition phase, as said signal is received,
• a module for comparing a received word with at least one received word of the same type received for at least one other of the transmitting satellites,

and, a module for validating or not synchronization with said identified transmitter satellite, depending on the result of the comparison, as soon as the comparisons made make it possible to reach a probability of false alarm and/or a predetermined probability of non-detection.

There picture 2 is a block diagram of the main steps of a synchronization validation method according to a first embodiment of the invention, implemented by a GNSS receiver 12.

This first embodiment applies in particular with the GPS system, in the context of a re-acquisition of a navigation signal for a given satellite Si of the constellation, after a loss or masking of the previously acquired navigation signal, on an associated channel.

It is assumed in this embodiment that one has ephemeris checked and stored in the memory 16 for the satellite Si, or, in other words, valid and non-obsolete ephemeris.

An ephemeris is made up of ephemeris data or words, each ephemeris word having an associated rank. For example, in a GPS system, there are 20 ephemeris words defined, with ranks (or indices) ranging from 1 to 20. We note m(i) the ephemeris word of rank i. The ephemeris word m(i) transmitted by a given satellite at a given instant has an associated value.

GPS signals are transmitted periodically and consist of pages, each page consisting of sub-frames, each sub-frame consisting of a given number of words, each word being coded on a given number of bits, transmitted at a frequency given. The words are protected by a protection code or error detection code, for example a parity, making it possible to easily detect any losses or errors at the transmission level.

The ephemeris words of given ranks are distributed in various subframes of the GPS signals.

In this first embodiment, the method comprises a first preliminary step 20 of obtaining a non-detection probability value P _nd , a parameter defined by an operator or as a function of security constraints of a targeted application.

Next, a step 22 of obtaining and storing probabilities is implemented so that at a given instant, an ephemeris word of the same rank is identical for two distinct satellites Si and Sj.

Let X(i) be the event associated with the fact that the word m(i) is identical in the ephemerides transmitted by two or more distinct satellites.

There are as many events X(i) as there are distinct ephemeris words.

, A∈ Ω _GPS,Pnd , les événements X(i) n'étant pas indépendants.In the preliminary step 20, an estimate of the probability P ( X ( i )) of each event X(i) is recovered, as well as of the set of probabilities $P\left({\displaystyle \underset{I\in \mathrm{AT}}{\cap }X\left(I\right)}\right)$

, A ∈ Ω _{GPS , Pnd} , the events X(i) not being independent.

Preferably, this estimate is made from databases archiving the existing ephemerides for the constellation considered.

The calculation of the probabilities is preferably done by a computer other than the geolocation receiver 12, and the values of the calculated probabilities are supplied to the receiver and stored by the latter.

The a priori estimation of the probabilities can be redone at any time to take into account any evolution of the GNSS system considered.

For the GPS system, whose ephemeris is made up of 20 distinct words, calculations on a long history of stored ephemerides have shown that the probabilities of the events X(i) are different for words of different ephemeris.

In addition, we estimate a priori, by estimation of the joint probabilities, the set Ω _Pnd of sets A of ephemeris words, designated by their respective ranks, for the GNSS system considered, making it possible to ensure with a probability of non-detection P _nd of given value, that these words cannot come from multiple satellites.

Mathematically we write: $${\mathrm{\Omega }}_{\mathit{pnd}}=\left\{\mathrm{AT}/\mathrm{AT}\subseteq \left\{1,\dots ,\mathrm{NOT}\right\},P\left(\underset{I\in \mathrm{AT}}{\cap }X\left(I\right)\right)<P_{\mathit{n/a}}\right\}$$

For example, the estimated probabilities are as follows for the first two ephemeris words, m(1) and m(2): P ( X (1)) = 2.8×10 ^-2 and P ( X (3) )=8.2×10 ^-3 , and for the set A = {1.3} the estimated probability is $P\left({\displaystyle \underset{I\in \mathrm{AT}}{\cap }X\left(I\right)}\right)=9.5\times {10}^{-4}$

Steps 20 and 22 are repeated, if necessary, for several given probabilities of non-detection P _nd .

For each of the values of P _nd given, the sets A of ranks of ephemeris words are stored, for example in the form of tables or any other suitable form.

It should be noted that the sets of indices A determined for a given probability of non-detection have different cardinals as a function of estimated probabilities P(x(i)), the probability of non-detection also depending on the ranks of the indices received . Thus, for example for the GPS system and a non-detection probability of 10 ^-4 , sets of one, two or three ephemeris words form the set $${\mathrm{\Omega }}_{\mathit{pnd}}=\left\{\mathrm{AT}/\mathrm{AT}\subseteq \left\{1,\dots ,\mathrm{NOT}\right\},P\left(\underset{I\in \mathrm{AT}}{\cap }X\left(I\right)\right)<P_{\mathit{n/a}}\right\}.$$

For each satellite Si considered, values of ephemeris words checked and validated are stored during step 24.

In one embodiment, the values are received by the receiver via another communication channel, for example by GSM radio communication or by communication via the Internet network.

As a variant, stored ephemeris word values have been received beforehand from the transmitter satellite Si and validated by a conventional validation method, for example the method of validation with respect to previously stored almanac data.

According to another variant, stored ephemeris word values have been transmitted by the satellite Si by a frequency other than the radionavigation signal to be processed, for example by the L5 signals, for which the risk of cross-correlation is very low.

, k indiquant le rang correspondant, i l'indice du satellite associé et N étant le nombre de mots d'éphémérides du système GNSS, par exemple N=20 pour le GPS.We denote the ephemeris words stored for a satellite Si: $M_I^m\left(k\right),k\in \left\{1,\dots ,\mathrm{NOT}\right\}$

, k indicating the corresponding rank, i the index of the associated satellite and N being the number of ephemeris words of the GNSS system, for example N=20 for the GPS.

During a step 26, a navigation signal Sig is received on the channel considered, and associated ephemeris words are extracted from this signal.

The signal Sig is considered to have been received from a transmitter satellite Sj following the signal acquisition phase comprising synchronization by correlation.

.These ephemeris words are called received words, and they are denoted: $M_I^r\left(k\right),k\in \left\{1,\dots ,\mathrm{NOT}\right\}$

.

The purpose of the method is to validate or not the synchronization with the signal Sj, in other words to validate, with an associated probability of non-detection, if the ephemerides received with the signal Sig are emitted by the signal Sj or if they are likely to be emitted by another satellite Si of the constellation.

To do this, step 26 of receiving and extracting ephemeris words received is followed by a step 28 of checking the parity of each ephemeris word received.

Indeed, as explained above, it is a protection mechanism having the objective of detecting possible transmission errors and therefore of avoiding the use of erroneous ephemeris words.

More generally, a verification of the protection code of the word received is applied.

In the event of negative verification, step 28 is followed by step 26 mentioned above.

avec le mot d'éphéméride de même rang mémorisé $M_j^m\left(k\right)$

, pour le satellite Sj identifié préalablement dans l'étape d'acquisition de signal de navigation.In case of positive verification, step 28 is followed by a step 30 of comparing each of the received ephemeris words extracted $M_I^r\left(k\right)$

with the ephemeris word of the same rank memorized $M_I^m\left(k\right)$

, for the satellite Sj previously identified in the navigation signal acquisition step.

It should be noted that the ephemeris words received are extracted from the sub-frames received progressively and processed, the purpose of which is to reduce the total synchronization validation time with the satellite Sj.

Thus, a subset of ephemeris words received is processed in step 28, this subset possibly being limited to a single ephemeris word.

, la valeur du mot reçu est bien égale à la valeur du mot mémorisé de même rang k, pour le satellite Sj. Comme expliqué ci-dessus, on dispose dans ce cas d'une estimation de la probabilité pour que le mot d'éphéméride reçu $M_j^r\left(k\right)$

provienne d'un autre satellite que le satellite Sj.Whether $M_I^r\left(k\right)=M_I^m\left(k\right)$

, the value of the received word is indeed equal to the value of the stored word of the same rank k, for the satellite Sj. As explained above, in this case we have an estimate of the probability that the ephemeris word received $M_I^r\left(k\right)$

comes from a satellite other than satellite Sj.

The result of the comparison is stored and accumulated with the previous results at processing step 32.

In one embodiment of the processing step 32, the rank k of the ephemeris word received and validated by the comparison is stored, and it is checked whether k belongs to one of the sets A of ranks of ephemeris words stored for reach the given non-detection probability P _nd .

The probability of non-detection P _nd is a parameter which is for example entered by an operator and supplied as input to the method or which is imposed by an application using the geolocation receiver.

For example, P _nd is 10 ^-3 or 10 ^-4 for usual applications.

In step 34 following step 32, it is checked whether one of the sets A of rows of ephemeris words has been completed, in other words whether it has been received and validated by comparison with the ephemeris words stored every words of a set A, thus ensuring that these words are sent by the transmitter satellite Sj considered with a probability of non-detection lower than P _nd .

In the event of a positive response, the validation of the synchronization with the transmitting satellite Sj is validated for the navigation signal Sig received. The algorithm ends (step 36). In the event of a negative response, step 34 is followed by step 26 of reception and extraction of ephemeris words previously described.

In the event of a negative response, when all the words of the GPS ephemeris are received, the receiver compares the decoded ephemeris to the almanacs to ensure the presence or absence of cross-correlation.

Thus, in the general case of absence of cross-correlation and absence of change of the ephemerides during the signal reception break, the validation of the synchronization is very fast. In the particular cases, of low occurrence, of change of ephemeris during the cut or of cross-correlation, the verification of all the words of the GPS ephemeris is carried out.

Advantageously, if the first ephemeris words received and processed during the application of this first moment of realization are validated and sufficiently distinctive in terms of probabilities, validation of the synchronization is very fast.

For example, with a probability of non-detection P _nd =10 ^-4 , synchronization validation, or confirmation of the absence of cross-correlation, takes 0.6 seconds in the best case, in which the first word of ephemeris received and processed is distinctive, and can take up to a maximum of 17.4 seconds.

In any case, validation is much faster than when classical and systematic validation is applied.

It should be noted that this first embodiment described above applies for constellations of satellites other than the GPS constellation, provided that sufficient data is available to obtain an estimation of the probabilities of the events X(i) a priori .

A second embodiment of the synchronization validation method with a transmitter satellite Sj, implemented by a GNSS receiver 12, is described with reference to the picture 3 .

This second embodiment differs from the first embodiment because it does not require an a priori estimation of the probabilities that two ephemeris words transmitted at the same time by two distinct transmitting satellites are identical.

In known manner, the GNSS receiver simultaneously receives and processes several reception channels, each reception channel being associated with a transmitting satellite during the navigation signal acquisition phase.

In this second embodiment, it is assumed that the processed navigation signal Sig is a sufficiently strong signal to generate a cross-correlation on a current reception channel, and that it is also received in the acquisition phase on another channel of the GNSS receiver.

In this second embodiment, a first step 40 is implemented on all the reception channels, to extract and store the ephemeris words received, each channel being associated with a transmission satellite.

This step is implemented following the prior acquisition of the navigation signal for each channel and comprises, for each reception channel, a first sub-step 42 of extracting and checking the parity of each ephemeris word received and a substep 44 of storing the ephemeris words whose parity has been checked, in relation to the reception channel processed.

où N est le nombre de mots d'éphéméride (égal à 20 pour le système GPS), et pour lesquels la parité a été vérifiée, afin d'éviter l'utilisation ultérieure de mots d'éphéméride erronés.Thus, for each reception channel associated with a transmitter satellite of index I, l ∈ {1,..., N _C } where Nc denotes the number of reception channels, ephemeris words are stored: $M_I^m\left(k\right),k\in \left\{1,\dots ,\mathrm{NOT}\right\}$

where N is the number of ephemeris words (equal to 20 for the GPS system), and for which the parity has been checked, in order to avoid the subsequent use of erroneous ephemeris words.

à l'étape 46.Substantially in parallel, and as reception progresses, when step 44 is implemented for a given processed channel, called the current channel, associated with a transmitter satellite Si, called the current satellite, for which synchronization validation, we obtain the received word whose parity has been checked $M_I^r\left(p\right)$

at step 46.

.From a set of stored ephemeris words associated with the transmitting satellite Si, the rank word corresponding to step 48, denoted $M_I^m\left(p\right)$

.

et $M_i^m\left(p\right)$

sont comparés à l'étape 50.Ephemeris words $M_I^r\left(p\right)$

And $M_I^m\left(p\right)$

are compared at step 50.

est comparé, lors de l'étape de comparaison 50, à tous les mots d'éphémérides de même rang et validés reçus sur les autres canaux de réception, notés $M_j^r\left(p\right),j\ne i$

.Moreover, the ephemeris word received $M_I^r\left(p\right)$

is compared, during the comparison step 50, with all the ephemeris words of the same rank and validated received on the other reception channels, denoted $M_I^r\left(p\right),I\ne I$

.

The results of the comparison are processed during a processing step 52.

et $M_i^m\left(p\right)$

sont identiques, et que le mot d'éphéméride $M_i^r\left(p\right)$

est différent de tous les mots reçus de même rang $M_j^r\left(p\right),j\ne i$

, sur un canal de réception différent, alors la synchronisation est validée pour ce mot, donc considérée comme non entachée de corrélation croisée.If the ephemeris words $M_I^r\left(p\right)$

And $M_I^m\left(p\right)$

are identical, and that the ephemeris word $M_I^r\left(p\right)$

is different from all received words of the same rank $M_I^r\left(p\right),I\ne I$

, on a different reception channel, then the synchronization is validated for this word, therefore considered as not vitiated by cross-correlation.

A validity counter, C _valid is incremented in step 54, this counter being initialized to 0 at each new implementation of the synchronization validation method.

et $M_i^m\left(p\right)$

sont différents, et que le mot d'éphéméride $M_i^r\left(p\right)$

a la même valeur qu'un ou plusieurs des mots reçus de même rang $M_j^r\left(p\right),j\ne i$

, sur un canal de réception différent, alors la synchronisation est invalidée pour ce mot, donc considérée comme entachée de corrélation croisée.If the ephemeris words $M_I^r\left(p\right)$

And $M_I^m\left(p\right)$

are different, and that the ephemeris word $M_I^r\left(p\right)$

has the same value as one or more of the received words of the same rank $M_I^r\left(p\right),I\ne I$

, on a different reception channel, then the synchronization is invalidated for this word, therefore considered as vitiated by cross-correlation.

An invalidity counter, C _invalid , is incremented at step 54, this counter being initialized to 0 at each new implementation of the synchronization validation method.

It should be noted that the validity counters C _valid and invalidity counters C _invalid are only incremented for words of distinct ranks. Thus, a word of the same rank received again cannot contribute to incrementing a non-zero counter.

It is then checked in a step 56 whether the validity counter or, if applicable, the invalidity counter, has reached a number of ephemeris words to be taken into consideration Nt.

This number Nt depends on the values of the probability of non-detection P _nd and of the probability of false alarm P _fa of a cross-correlation.

When the number Nt is reached, the synchronization validation process for the navigation signal received on the current channel ends.

If the number Nt is not reached, step 56 is followed by step 46 previously described for processing a next ephemeris word for the channel associated with the transmitter satellite Si.

If the validation of the synchronization is negative, advantageously, the re-acquisition of the synchronization can be restarted without waiting, which makes it possible to improve the radionavigation.

The number Nt of ephemeris words to be taken into account is, in one embodiment, equal to 1.

For a given received ephemeris word, the associated non-detection probability and false alarm probability can be calculated.

Regarding the probability of non-detection, there is a risk of non-detection if a word is decoded incorrectly. The probability of non-detection due to such an error is dependent on the word error rate, also called WER for “word error rate”, which depends on the signal-to-noise ratio of the signal received, falsely considered valid.

For a GPS word coded on 24 bits and protected by 6 parity bits, we have: P _nd < WER ×2 ^{- 6} ×2 ^-24 , where 2 ^-6 is the error probability for a word protected by 6 bits of parity and 2 ^-24 is the probability that the word received is equal to a word received for another satellite.

The probability of false alarm, which is the probability of false detection of a cross-correlation phenomenon, is calculated, according to one embodiment, with respect to the renewal of the ephemeris sent with respect to the ephemeris stored by the same satellite, renewal which is carried out with a given frequency, for example 2 hours.

The number Nt of ephemeris words to be taken into consideration is determined differently depending on whether the processing knows that the ephemeris sent have changed or not with respect to the stored ephemeris. One of the ephemeris words makes it possible to date it. If it is received and equal to that of the stored ephemeris, Nt will be determined on an assumption of no change, if it is not yet received or if it is received different from that of the stored ephemeris , Nt will be determined on a change assumption.

Indeed, if the sent ephemerides have not changed compared to the stored ephemeris for a given transmitter satellite, the probability of false alarm is calculated according to the probability of decoding error WER, parity error (2 ^{- 6} ) and equality of the decoded word with respect to one of the words of the same rank decoded on one of the other satellites (1-(1-2 ^-24 ) ^{N vs -1} ).

If the receiver receives Nc satellites, we have: $$P_{\mathit{ƒa}}<\mathit{WER}\times 2^{-6}\times \left(1-{\left(1-2^{-24}\right)}^{{\mathrm{NOT}}_{\mathrm{vs}}-1}\right)$$

For Nc=10, there is a probability of false alarm well below 10 ^-8 .

If the ephemeris sent has changed from the ephemeris stored for a given transmitter satellite, the risk of a false alarm is greater.

Let Y(i) be the event associated with the fact that the words m ( i ) of two ephemeris tables transmitted successively by the same satellite are identical.

We then have: $$P_{\mathit{ƒa}}=P\left(\underset{I\in \mathrm{AT}}{\cap }Y\left(I\right)\right)\left(1-{\left(1-P\left(\underset{I\in \mathrm{AT}}{\cap }X\left(I\right)\right)\right)}^{{\mathrm{NOT}}_{\mathrm{vs}}}\right)\approx {\mathrm{NOT}}_{\mathrm{vs}}\cdot P\left(\underset{I\in \mathrm{AT}}{\cap }Y\left(I\right)\right)\cdot P\left(\underset{I\in \mathrm{AT}}{\cap }X\left(I\right)\right)$$

A being the set of ranks (or indices) of the ephemeris words received verifying the invalidation criterion, and Nc the number of transmitting satellites received.

For example, if we want a false alarm probability P _fa <10 ^-3 , and Nc=10, we use the sets of indices A which provide an a priori estimate of the associated probability of 10 ^-4 , as explained below. above compared to the first embodiment.

se fait de la même manière que pour déterminer les probabilités $P\left({\displaystyle \underset{i\in A}{\cap }X\left(i\right)}\right)$

: cette estimation est effectuée à partir de bases de données archivant les éphémérides existantes pour la constellation considérée.The event probabilities Y(i) are estimated a priori, on another computer, solely to determine the value Nt stored by the receiver as a function of Nc. The determination of probabilities $P\left({\displaystyle \underset{I\in \mathrm{AT}}{\cap }Y\left(I\right)}\right)$

is done in the same way as to determine the probabilities $P\left({\displaystyle \underset{I\in \mathrm{AT}}{\cap }X\left(I\right)}\right)$

: this estimate is made from databases archiving the existing ephemerides for the constellation considered.

As in the first embodiment described above, a priori estimates of the probabilities can be used.

It should be noted that in this case, the number Nt of ephemeris words to be taken into consideration for a given probability of false alarm also depends on the ranks of the indices received, sets of one, two or three ephemeris words , being part of the set e defined in equation (Eq 1) above: $${\mathrm{\Omega }}_{\mathit{pnd}}=\left\{\mathrm{AT}/\mathrm{AT}\subseteq \left\{1,\dots ,\mathrm{NOT}\right\},P\left(\underset{I\in \mathrm{AT}}{\cap }X\left(I\right)\right)<P_{\mathit{n/a}}\right\}.$$

As a variant, if an a priori estimate of the probabilities is not available, it suffices, when renewing the ephemeris sent by a satellite, to consider all the ephemeris words received. In this variant, no a priori estimation of probability is necessary.

The method proposed above nevertheless applies and makes it possible to considerably accelerate the validation of the synchronization throughout the operating phase with the same set of ephemeris data.

The method proposed in this second embodiment above applies analogously to the GALILEO L1 BC radionavigation signals.

The GALILEO L1 BC signals have a periodic code of 4092 elements, but do not however allow sufficient robustness to guarantee the absence of cross-correlation.

The method described above applies with the ephemeris data of the GALILEO system, taking into account the specific arrangement of the I/NAV navigation messages and the protection code which is a CRC code (for "redundancy check cyclic”), applied to a plurality of transmitted ephemeris words.

It should be noted that the ephemeris word of rank 4 in the GALILEO system carries the identifier of the transmitting satellite, therefore error-free decoding of this ephemeris word makes it possible to identify the transmitting satellite.

However, ephemeris words are transmitted cyclically, so it is possible to have to wait up to 30 seconds to decode the rank 4 ephemeris word.

If faster synchronization validation is desired, the method of the invention described above applies, and in particular the second embodiment which does not require a priori calculation of probabilities associated with the ephemeris words transmitted by the different satellites of the constellation.

The calculations of probability of non-detection P _nd and of false alarm P _fa are adapted to the case of the GALILEO system.

For example, for an ephemeris word protected by a 24-bit CRC code, and coded on 110 bits, we have: $$P_{\mathit{n/a}}<\mathit{WER}\times 2^{-24}\times 2^{-110}\ll 4\cdot {10}^{-41}$$

Advantageously, the use of a single ephemeris word to validate or invalidate the presence of a cross-correlation provides a high level of integrity, which limits the validation time from 2 seconds minimum to 18 seconds maximum.

According to a third embodiment, the invention also applies to the case of augmentation signals, for example SBAS, comprising geolocation correction and integrity data for a predetermined satellite geolocation system, in the case receivers adapted to receive several SBAS signals on several reception channels, and therefore are liable to undergo the phenomenon of erroneous synchronization on a transmitting SBAS satellite, due to cross-correlation.

Each reception channel is associated with a transmitting SBAS satellite.

SBAS signals are sent by service providers, who generally own several satellites. For example, we know the service providers EGNOS ^® for Europe and WAAS ^® for the United States, GAGAN ^® for India, MSAS ^® for Japan.

Each service provider's satellites send ephemeris, but only at lower time frequencies than geolocation satellites.

In addition, each service provider broadcasts almanacs of the satellites in its constellation.

There figure 4 is a block diagram of the main steps implemented in the third embodiment of the method for validating synchronization with a transmitter satellite Sj of an SBAS-type spatial augmentation system, implemented by a receiver capable of receiving SBAS type signals on several reception channels.

The method of the invention is suitable for this scenario, in which the sending of ephemerides is grouped and sent at low frequency (for example every 120 seconds for SBAS).

In the SBAS system, each word, regardless of type, consists of 250 bits and is protected by a 24-bit CRC protection code.

During a first step 60, the almanacs of the various service providers are received and stored.

This step 60 is implemented by any appropriate means of communication, for example by using another communication channel—GSM, Internet or another appropriate communication channel.

Step 60 is followed by a step 62 of receiving SBAS signals after acquisition or re-acquisition on each of the reception channels processed, and of extracting a current word received having an associated type, then by a step 64 verification of the CRC code of the word received.

In case of negative verification at step 64, therefore if there is a possible reception error on the word received, step 64 is followed by the previous step 62 of reception and extraction of a new word received.

In the event of positive verification at step 64, the word received is stored in relation to the associated channel in order to then carry out comparisons between words received.

During processing step 66, if the word received corresponds to the reception of the ephemeris word, then step 66 is followed by step 70 described below, otherwise, the word received on the current reception channel is compared to the received words of the same type on the other reception channels.

The step 70 which corresponds to the reception of the ephemeris word consists of the comparison of the ephemeris with the almanac data previously stored for the current service provider.

The comparison will then reveal the existence of a cross-correlation.

In the event of a positive comparison in step 66, therefore if the current word received, originating from the current reception channel, called the first channel, is identical to a word received from a second channel, and the type of the word is different from the type 0 in the SBAS system where type 0 words have fixed content, then step 66 is followed by step 68.

In step 68 it is checked whether the first channel and the second channel are both associated with satellites of the same service provider, called the current service provider.

If yes, step 68 is followed by step 62.

In the event of a negative response at step 68, therefore if the first channel and the second channel are associated with satellites of different service providers, an invalidity counter C _invalid is incremented. Step 68 is followed by step 74 described below.

As in the second embodiment, the invalidity counter, C _invalid , is initialized to 0 at each new implementation of the synchronization validation method.

If in step 66 the current word received, from the current reception channel, called the first channel, is different from all the words of the same type received from all the other channels, then the signal is considered valid and therefore not tainted by a cross-correlation.

A validity counter C _valid is incremented in step 72, this counter being initialized to 0 at each new implementation of the synchronization validation method.

Step 72 is followed by a step 74 of comparing the validity counter or, where applicable, the invalidity counter, with a number of words received to be taken into account Nr.

This number Nr depends on the values of the probability of non-detection P _nd and of the probability of false alarm P _fa of a cross-correlation.

When the number Nr is reached, the synchronization validation process for the signal received by the spatial augmentation system on the current channel ends.

If the number Nr is not reached, step 74 is followed by step 62 previously described for processing a next received word.

The number Nr of words received to be taken into account is, in one embodiment, equal to 1.

For a given word received, it is possible to calculate the probability of non-detection and the probability of false alarm of an associated cross-correlation.

Regarding the probability of non-detection, there is a risk of non-detection if a word is decoded incorrectly. The probability of non-detection due to such an error is dependent on the word error rate, also called WER for “word error rate”, which depends on the signal-to-noise ratio of the signal received, falsely considered valid.

For an SBAS word coded on 250 bits and protected by a 24-bit CRC, we have: P _nd < WER ×(1-(1-2 ^-24 ) ¹²⁰ )<<7 10 ^-6 . The use of a single received word to confirm the absence of cross-correlation provides a high level of integrity.

There is a risk of false alarm if two words transmitted by two satellites from different service providers are identical.

The false alarm probability, which is the probability of false detection of a cross-correlation phenomenon, is calculated, according to one embodiment, by: $$P_{\mathit{ƒa}}<2^{-218}<2\cdot {10}^{-66}$$

Using a single word to confirm the presence of cross-correlation brings a low risk of false alarm.

Advantageously, in all the embodiments described, the validation or invalidation of the synchronization with a transmitting satellite is accelerated thanks to the use of received words received for the same transmitting satellite previously identified in the acquisition phase, and, if applicable, for other transmitter satellites of the constellation which are received by the geolocation receiver considered.

In the case of spatial augmentation systems, these are words received without distinction of type, whereas for GNSS radionavigation systems, these are ephemeris words.

Claims (9)

1. A method for validating the synchronization between a geolocation receiver and a transmitting satellite during a phase for acquiring a navigation signal from this satellite,

   said receiver being able to receive a composite radio signal including a plurality of navigation signals each transmitted by a transmitting satellite that is part of a satellite constellation, and to implement a phase for acquiring a navigation signal for each of the transmitting satellites, in which the receiver determines a satellite transmitting a corresponding navigation signal in a synchronization step,

   each transmitting satellite further transmitting sets of ephemerids made up of a plurality of words, each having an associated rank, encoding information relative to said transmitting satellite and making it possible to compute the position of said transmitting satellite in a given plane of reference, the method being implemented by said receiver,

   wherein the method comprises :
   for each identified transmitting satellite, performing a validation test, comprising:

   - extracting (26, 42) ephemerid words from the navigation signal associated with the transmitting satellite identified in the acquisition phase, as said navigation signal is received,

   - comparing (30, 50) at least part of said extracted ephemerid words to at least one stored ephemerid word with the same rank for said identified transmitting satellite and/or for at least one of the other transmitting satellites, and

   based on the result of the comparison,

   verifying a synchronization validation condition (32-34, 52-56) validating the synchronization with said identified transmitting satellite, with a predetermined false alarm probability and/or non-detection probability,

   the false alarm designating a false detection of a cross-correlation,

   the non-detection designating a non-detection of cross-correlation, the probability of non-detection being estimated as a function of

   the probability that at least one ephemerid word of same rank is identical in the ephemerids transmitted for two or more distinct satellites, or

   the probability that said ephemerid word received by said transmitting satellite is identical to a word received for another satellite, and

   in case of positive verification, stop the validation test,

   in case of negative verification, return to the extracting step.
2. The method according to claim 1, characterized in that it comprises, after the extraction step, a step for verifying (28, 44) a protection code associated with one or several of said extracted ephemerid words, and wherein the comparison (30, 50) is done on the extracted ephemerid words for which the verification is positive.
3. The method according to one of claims 1 or 2, characterized in that it comprises a prior step for estimating (20, 22) a probability, for each ephemerid word of the set of ephemerids, that an ephemerid word, having an associated rank and transmitted by a given transmitting satellite, has a same value as a transmitted ephemerid word with the same rank transmitted by another satellite in the satellite constellation.
4. The method according to claim 3, characterized in that it further comprising computing (22) sets of ranks of ephemerid words, each said set of ranks including ephemerid word ranks allowing validation of the associated transmitting satellite with a corresponding non-detection probability, and storing (22) at least one set of computed ephemerid rank sets corresponding to a predetermined non-detection probability.
5. The method according to claim 4, characterized in that it comprises, in case of positive comparison of a received ephemerid word with an ephemerid word previously validated and stored for said transmitting satellite, a validation (34) based on whether the rank of said received ephemerid word belongs to at least one of said computed sets of ephemerid ranks.
6. The method according to one of claim 1 or 2, characterized in that it further comprises comparing (50) an extracted ephemerid word with a stored ephemerid word having the same rank for said identified transmitting satellite, and with an ephemerid word of the same rank received for each of the plurality of other transmitting satellites.
7. The method according to claim 6, characterized in that it :

   - if the extracted ephemerid word is identical to the stored ephemerid word having the same rank for said identified transmitting satellite, and different from each of the ephemerid words having the same rank received for another transmitting satellite, a validity counter is incremented (54), or

   - if the extracted ephemerid word is different from the stored ephemerid word having the same rank for said identified satellite, and identical to at least one of the words having the same rank received for another transmitting satellite, an invalidity counter is incremented (54).
8. The method according to claim 7, wherein a validation or invalidation of the synchronization is done based on the comparison (56) of the validity counter or invalidity counter to a number of comparisons to be taken into consideration, said number of comparisons to be taken into consideration depending on the predetermined false alarm probability and/or non-detection probability.
9. A geolocation receiver performing a synchronization and a transmitting satellite during a phase for acquiring a navigation signal from this satellite, said receiver being able to receive a composite radio signal including a plurality of navigation signals each transmitted by a transmitting satellite that is part of a satellite constellation, and to implement a phase for acquiring a navigation signal for each of the transmitting satellites, in which the receiver determines a satellite transmitting a corresponding navigation signal in a synchronization step,

   each transmitting satellite further transmitting sets of ephemerids made up of a plurality of words, each having an associated rank, encoding information relative to said transmitting satellite and making it possible to compute the position of said transmitting satellite in a given plane of reference,

   wherein the receiver comprises

   modules for performing a validation test for each identified transmitting satellite, comprising:

   - a module for extracting ephemerid words from the navigation signal associated with the transmitting satellite identified in the acquisition phase, as said navigation signal is received.

   - a module for comparing at least part of said extracted ephemerid words to at least one stored ephemerid word with the same rank for said identified transmitting satellite and/or for at least one of the other transmitting satellites, and

   - a module for validating or not validating the synchronization with said identified transmitting satellite,

   implementing, based on the result of the comparison, verifying a synchronization validation condition validating the synchronization with said identified transmitting satellite, with a predetermined false alarm probability and/or non-detection probability,

   the false alarm designating a false detection of a cross-correlation,

   the non-detection designating a non-detection of cross-correlation, the probability of non-detection being estimated as a function of

   the probability that at least one ephemerid word of same rank is identical in the ephemerids transmitted for two or more distinct satellites, or

   the probability that said ephemerid word received by said transmitting satellite is identical to a word received for another satellite, and

   the receiver being configured :

   in case of positive verification, for stopping the validation test,

   in case of negative verification, for extracting ephemerid words from the navigation signal associated with the transmitting satellite identified in the acquisition phase, as said navigation signal is received.

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